1. Field of the Invention
The present invention relates to a D.C. fan control circuit device for linearly variable cooling, and more particularly to a device for cooling a computer equipment or the like.
2. Description of the Related Arts
In recent years, it is required that a computer equipment should be operated in a low noise condition. For this reason, it is considered that a noise of a cooling fan as one of sound sources is reduced. As one of the noise reduction means, it is a recent trend that a temperature within the computer equipment or in an environment of the equipment is detected to make a revolutional speed of a fan change linearly and a low revolutional speed of the fan in low temperature keeps a noise in a low state to decrement an average noise.
When a computer equipment is cooled by the D.C. fan, if the D.C. fan is always driven at a constant speed, it is necessary to effect a high speed operation in conformity with a high temperature state of an environment around the computer equipment. Thereby, a high speed operation causes undesired high sound.
Therefore, two-speed type cooling system of the fan drive unit is adopted such that, when the environment is at a low temperature, the D.C. fan is driven at a low speed to cause a still state, whereas when the environment is at a high temperature, the D.C. fan is driven at a high speed to improve a cooling efficiency.
Nevertheless, according to the two-speed type cooling system, when the D.C. fan is driven near a switching-over temperature that converts a low speed drive to a high speed drive or vice versa, since a switching takes place frequently due to the up-down changes of temperature, the undesired sound of the D.C. fan varies frequently in proportion to the number of times of switching to cause a jar and become offensive to the ear.
As a result, it is desired that, when the temperature is more than a predetermined value, the D.C. fan is run fast at a definite rotational number, and when the temperature is less than a predetermined value, the rotational number is varied linearly to cause a noise at a lower level and make inoffensive to the ear.
When tried, it is found that the countermeasure does not work very well. To be more specific, multifarious disadvantages will be described as follows.
In the prior art linear speed control cooling system for the D.C. fan, if a driving voltage for the D.C. fan is accidentally applied to a lead to which a temperature sensor will be connected, it happens that a driving IC is damaged. Therefore, when a driving voltage is accidentally applied to the lead line, the driving IC is damaged and cannot make the D.C. fan rotate. Thereby, it will be disadvantageous that the computer unit is not cooled down to effect normal operation.
Since the computer equipment is designed with an originally forced air cooling system using a fan, when a rotation of the fan is stopped to be in a natural air-cooled state, the temperature of a specified part within the equipment rises extraordinarily. From this heating, it is disadvantageous that the computer equipment cannot be operated ordinarily or a system failure occurs.
Therefore, an object of the present invention is to provide a control circuit of the D.C. fan for linearly variable cooling that reduces or eliminates the above-described problems.